He knew Misa would be coming to fuss at him to sleep. He knew it would likely lead to disagreement - Vader not wanting to sleep, Misa knowing he needed to. And eventually, he would doze, in fits and starts. But he was unwilling to sleep, here, on Endor, with it being so unstable.

Not that the girl hadn't kept watch before, not that he didn't trust her to do so, over himself and his son. He's not paranoid, people truly want him dead. Would harm her. Would possibly harm Luke to get through.

But Luke at least needs to sleep and he knows Misa won't leave him until he is.